Hydrostatically powered driven equipment such as lawn tractors have become extremely popular and many utilise the axial piston swash-plate configuration for the pump and motor elements of the hydrostatic transmission. Such tractors generally have an internal combustion engine having a vertical crankshaft which is connected to the transaxle power input by means of a conventional belt and pulley arrangement whereby engine power is transmitted by the belt to an input drive shaft supported in the transaxle housing. The input shaft is operatively connected to the pump to cause the pump to rotate at high speed as shown in U.S. Pat. Nos. 5,090,949 and 5,771,758 both of which are incorporated herewith as reference. In these particular examples of integrated hydrostatic transaxle apparatus, the axial piston swash-plate pump is fluidly coupled to the fixed-displacement axial piston motor by means of a valve member, most often referred to as a center section, and where the center section is internally disposed within a chamber formed by surrounding housing of the hydrostatic transaxle and fixedly attached in some manner to the housing. The chamber serves as the fluid reservoir for the hydrostatic transmission whereby the rotating elements of the pump and motor as well as other elements such as the center section remain fully immersed in the power transmission fluid contained within the chamber.
The cylinder-barrel of the pump, being operatively connected to the input drive shaft, rotates at high speed and when the variable angle swash-plate is inclined with respect to the rotational axis of the cylinder-barrel, axial sliding motion of the pistons occurs within their respective cylinders causing fluid to be displaced within said cylinders. The fluid flow thus created by the reciprocating axial motion of the pistons is channeled via porting and passages in the center section to the hydraulic motor, with the effect that the incoming fluid causes the pistons of the motor to reciprocate and create a turning moment that causes rotation of the hydraulic motor. The hydraulic motor in turn has an output shaft which drives the vehicle's axles through speed-reducing gears and a mechanical differential.
The center section provides support surfaces for the pump and motor as well as providing within its interior structure the necessary fluid link in the form of passages so that hydraulic power can be transmitted between the pump and motor. The center-section as shown in these references require extensive machining operations in order to convert the initial raw casting into the ready to be used component. Apart from machining some of the internal passages and fixing points, the external faces on which the cylinder-barrels of the pump and motor operate against must also be prepared. Internal valves are fitted in some of these machined passages, for instance, fluid make-up check-valves, and the center-section must be designed so that it can be readily assembled and attached to interior wall structure of the transaxle housing. Mounting surfaces in the transaxle housing interior must also be machining in preparation for the attaching and fixing on the center section. The housing furthermore, requires a number of other machining operations, for instance finish sizing in the raw casting of the various apertures and holes for bearings and seals required for the drive shaft and speed control mechanism. In general terms, the more components requiring machining, the most costly the manufacture of the transaxle. Therefore there would be savings if many or all of the above referred to machining operations could be grouped into a single component, preferably in that component containing the fluid passages as well as the bearings and seals for the drive-shaft and speed control mechanism, and there would be an attendant saving in assembly as components such as shafts, valves would be first assembled in a sub-assembly.
Although only shown in the '949 patent, almost all hydrostatic transaxles make use of a cooling fan mounted to the input drive shaft in an attempt to help prevent the internal components and fluid from overheating. However, the prior art teaches a center section which although attached in some manner to the interior of the housing, it is still essentially a separate entity from the transaxle housing. As a result, effective cooling of the fluid passing through the passages in the center section that connect the pump and motor together is hindered as the fluid surrounding the center section acts as a insulating medium to slow down the rate of heat transfer from the power transmitted fluid in said passages to the surrounding housing radiator.
The amount of heat able to be radiated away from the transaxle housing exterior to the surrounding environment is of course greatly enhanced over that region on the boundary of the transaxle housing that lies directly in the path of the air flow from the cooling fan. However, it is apparent that although the fluid inside the housing nearest that region where the fan is operating is being cooled, fluid elsewhere may still remain at very high temperature. Perhaps more importantly, as the fluid circulating between the pump and motor in the fluid passages in the center section becomes extremely hot during operation, especially when the unit is heavily loaded and used in a high ambient temperature environment, the resulting drop of operating efficiency due to decreasing fluid viscosity and a corresponding increase in fluid leakage losses can be a concern with the prior art.
This problem exists because the attendant power losses associated with such close coupled pump and motor combinations produce a lot of unwanted heat due to the rapid fluid compression/decompression cycles and general friction between the sliding surfaces. Such losses causes the fluid circulating between the pump and motor through the center section to become extremely host, and because the prior art teaches a transaxle housing structure whereby the internal fluid reservoir completely or almost completely surrounds and insulates the center section, these prior solutions are not conductive to the promotion of most effective cooling for the circulating fluid in the centre section flowing in a closed-loop circuit between the pump and motor. This limitation occurs because the bulk of the heat accumulating in the center section can only be transferred by conduction to the surrounding hydraulic fluid and then through the fluid itself to reach the boundary walls of the housing surrounding the fluid chamber from where it can be radiated away to the surroundings. The remove of unwanted heat from the center section consequently takes time.
Therefore in these prior devices where the center section is effectively insulated by the surrounding hydraulic fluid medium, the delay in the transfer of unwanted heat out of the transaxle may on occasion result in the fluid of the hydrostatic transmission becoming overheated with the risk that the operational life of the fluid is shortened or that the lubricating properties of the fluid deteriorates to the extent that threatens the useful operational life of the hydrostatic transaxle.